1. Field of the Invention
The present invention relates to devices for ultrasonic imaging of hollow objects. More particularly, the present invention relates to devices for controlling the positioning and rotation of mirrors used to transmit and receive ultrasonic signals. Still more particularly, the present invention is related to magnetically-operated devices for ultrasonic intravascular imaging. The present invention includes methods and systems for levitating and positioning objects.
2. Description of the Prior Art
The use of ultrasonic imaging systems for evaluating the interior of hollow objects is well known. In the medical field in particular, such devices have been employed for the purpose of examining the condition of a patient. Of necessity, such devices must be very small and essentially self-contained. For the most part, these devices include a transducer that transmits ultrasonic signals and a receiver that recovers the response signal. The transducer and receiver may be separate components, or they may be incorporated as part of a single crystal.
The devices used in ultrasonic imaging within a hollow object sweep the "beam," or line of sensitivity for transmission and reception, through an arc or circle, either by phased array techniques, or by selection from among multiple transducers differently aimed, or by rotation of a single transducer, or by rotation of a mirror that reflects and sweeps the alignment of a single-transducer beam. The purpose of sweeping is to provide a sectional view into and beyond the walls of the hollow object. Among these sweeping techniques, the phased array approach is the most expensive, difficult to miniaturize, and demanding of multiple connections between electronic and electro-acoustic areas. These multiple connections imply cabling or intimate integration of active semiconductor devices with metalizations driving and sensing separate regions of a transducer with differing phase responses. It is impractical, in a very small intravascular transducer, to provide many broadband transmission lines, low in crosstalk, within a cable, and it is similarly impractical to shrink an integrated semiconductor-transducer device to the needed size. Selection from among multiple transducers entails cabling problems and severe constraints on the size of individual transducers, implying problems with directional focusing and sensitivity. Single-transducer approaches offer the best possibilities for a transducer of maximum size and aperture, to achieve high sensitivity and directionality, within a constrained overall package size. Among these approaches, the rotatable mirror avoids the problems of commutation of the ultrasound electric signal from circuitry to the rotating transducer.
A well-explored approach with a single transducer is the "speedometer cable" configuration, locating a transducer or mirror at the distal end of a cable that rotates inside the lumen of a catheter. The effectiveness of this approach declines at very small catheter diameters and where the catheter must follow a serpentine path through human vasculature. Under these conditions, a uniform rotation driven at the proximal end results in a very non-uniform rotation at the distal "business" end as the cable binds, becomes twisted, and then slips to give a sudden angular whip. Thus, it has proved impractical to shrink coronary artery imaging transducers below an outside diameter of 0.040", and even on that scale, performance is limited.
Rotation drive by a micromotor has been explored using electric and hydraulic means. In the hydraulic motor approach, a rotary impeller is driven by fluid flow through the catheter, typically rotating an ultrasound mirror, so that electrical commutation of the transducer signal is avoided. To obtain consistent indication of the angular alignment of the mirror, it is necessary to provide a rotation encoder, which adds expense and difficulty in miniaturization. In the electric motor approach, a physical constraint is electromechanical efficiency, which inherently declines as a function of physical dimensions, commonly falling below 1% on the scale of a coronary artery device. On this scale, even a small bearing friction stops rotation or demands a power input sufficient to burn up the motor. The extremely low rotational inertia of a micromotor implies that angular momentum does not smooth out rotation, so that very small bearing nonuniformities result in jerky rotation. A small particle of dust will jam a bearing. As a result of these difficulties of small scale, even expensive tooling and techniques for maintaining extremely tight geometric tolerances have proved inadequate.
While magnetic levitation approaches may have been considered by researchers in an attempt to overcome the bearing friction and geometric tolerance problems of micromotors, it might appear that the physics dictating declining electromechanical efficiency with declining scale would preclude magnetic levitation on a very small scale, barring body-temperature superconductors, because such devices would burn up generating the needed levitation forces. It is known from physics that, barring superconductors, the passive interactions of permanent magnets cannot result in simultaneous position stabilization with respect to three orthogonal position axes. It has not been obvious that position-correcting control forces can be used dynamically to correct the position of a micromotor rotor to that unstable and time-varying position, in the force field set up by passive permanent magnet interactions, where the passive forces counterbalance the gravitational and acceleration forces exerted on the rotor. The force perturbations generated by electric currents then serve only to correct transient position errors, along one or two unstable axes, arising from changes in externally-imposed accelerations, from servo transducer noise, and from the inherent passive tendency toward exponential growth of position errors. The forces required to launch a rotor to a levitated position of passive dynamic force balance are then of such short duration that the thermal capacity of magnetic coils safely absorbs the transient energy. The practical implementation of such an approach, leading to a simple mechanical and electronic design with forgiving tolerances and low tooling and fabrication costs, is a needed breakthrough for intravascular imaging, as realized in the invention to be described herein.